350 



PRINCIPLES OF ANIMAL BIOLOGY 



Hviids. The embryo, however, is a three-cornered httle animal with 

 jointed legs which clearly marks Sacciilina as one of the Crustacea. It is, 

 in fact, one of the barnacles, a group in which adult structure is usually 

 quite comphcated (Fig. 242). Its ancestors, if we may assume evolution 

 to explain this case, were as highly developed as were other barnacles, 

 but by a process of retrograde evolution it has become the degenerate 

 mass of pulp that it is today. Comparative embryology also reveals, 

 not only the fact of evolution, but the course of evolution in some cases. 



Fig. 241. — Sacculina, parasitic on crabs. A, young Sacculina, shortly after hatchingi 

 sho-wing that its early development is like that of the Cirripedia (barnacles, etc.). B< 

 young animal shown attached to its host, the crab. The projection at the anterior end has 

 penetrated the chitinous ventral wall of the abdomen of the crab, only a small piece of the 

 chitin being shown. Most of the early structure of the parasite is already lost. The mass 

 of cells here shown subsequently passes through the tubular passage in front into the crab's 

 body. C, adult Sacculina (s), consisting of a pulpy mass on the under side of the crab's 

 abdomen, and a host of branching processes in the host's body. These processes absorb 

 nutriment from the body fluids of the host. Practically all of the structure characteristic 

 of the barnacles is lost. Magnification is not the same in the three figures. (A and B 

 after Delaue.) 



A case in point is the development of gill bars in the embryos of all ver- 

 tebrate animals, whereas in only a few of the groups do adults have gills. 

 Gill bars in the embryos not only reveal common ancestry, and hence a 

 subsequent evolution, but also indicate that the common ancestors were 

 fish-like animals — fish-like at least to the extent of having gills. 



Comparative Physiology. — Similarities in physiological features 

 are to be explained as a result of common descent, but known cases of 

 this kind are less numerous than easels of morphological likeness, probably 

 owing to the difficult}^ of discovering them. Animals whose blood com- 

 positions are very similar, as shown by the results of blood transfusions, 

 are assumed to be closely related. At any rate, similar blood would 

 be a result of common ancestry, and whatever differences now exist are 

 the result of evolution. Fortunately, in general, animals shown by blood 



